The Epidermal Growth Factor Receptor (EGFR) and its downstream signaling pathways, primarily the mitogen-activated protein kinase (MAPK) and the phosphatidyl-inositol-3 kinase (PI3K)/Akt pathways, are known to play a significant role in tumour growth and progression. Over-expression of the EGFR has been identified in numerous solid tumours including colon, rectal, lung, ovarian, and head and neck cancers. Many clinical studies have indicated that over-expression of EGFR in these tumours is associated with poor prognosis, and shorter overall survival of subjects afflicted with cancer.
As EGFR signalling appears to have limited relevant physiological function in an adult, provision of inhibitors of EGFR signalling is believed to allow selective targeting of malignant cells in a subject afflicted by cancer, whilst only causing limited toxicity to normal cells. EGFR has become a widely studied molecular target and major pharmaceutical companies have developed either small molecule inhibitors to EGFR such as Gefitinib (Astrazeneca) and Tarceva (OSI) or antibodies that exhibit high affinity binding and inhibit ligand-induced activation of the EGFR, such as Cetuximab (Eli Lilly/BMS/Merck Serono) and Panitumimab (Amgen).
Clinical trials of these EGFR targeted therapeutics have indicated that only subsets of patients actually derive clinical benefit from the provision of EGFR-targeted therapeutics. EGFR is expressed in 30-85% of colorectal cancers and its expression level has been linked to reduced survival. Trials conducted in patients with metastatic colorectal cancer have reported favourable response rates in only 23% of patients, even when the EGFR-therapeutic is combined with chemotherapy, and significantly lower response rates of only 11% when the EGFR-therapeutic in used as monotherapy (Saltz et al. J Clin Oncol 2004;22:1201-1208; Cunningham et al., N Engl J Med 2004;351:337-345). Identifying which patients respond favourably to the provision of EGFR-therapeutics has thus been a subject of intense research. Importantly, such predictive biomarkers of patient response to EGFR-therapeutics hold the promise to not only improve treatment outcomes for specific patients, but also will enable the full promise of these molecular-targeted therapeutic agents be realized.
Several early candidates were proposed to serve as predictive biomarkers of response to EGFR-therapeutics, including the expression level of the EGFR itself. Investigations in metastatic colorectal cancer however failed to demonstrate a correlation between EGFR expression as measured by immunohistochemistry and the clinical response to Cetuximab in combination with irinotecan chemotherapy (Chung et al, J Clin Oncol. 2005;23:1803-1810). Alternatively, while the use of activating mutations in the kinase domain of the EGFR have been successful to predict response to the provision of small molecule agents targeting the kinase domain of the receptor, these mutations are unable to predict response to antibodies such as Cetuximab that bind to the ectodomain of the receptor. Furthermore, mutation rates of the EGFR in certain diseases including colorectal cancer are low, and in diseases where mutations of the EGFR are present, these represent only small populations of the entire patient population (typically <10%) (Janne et al. J Clin Oncol 2005;23:3227-3234).
More recent attention has focused on identifying mutations resident within the constituent proteins of the signal transduction pathways that are activated by the EGFR. Following a number of Phase III clinical trials conducted in advanced stage colorectal cancer disease, Cetuximab has been approved for the treatment of subjects with epidermal growth factor receptor (EGFR)-expressing, KRAS wild-type metastatic, colorectal cancer (mCRC), in combination with chemotherapy, on the basis that tumours that do not harbour mutations are more likely to respond to EGFR-targeted treatment strategies. However, larger trials that have enrolled significantly larger numbers of patients have subsequently reported that use of KRAS status does not clearly predict response to EGFR therapeutics. For example, the randomized phase III COIN study, in which greater than 2500 patients took part, failed to observe any benefit from the addition of Cetuximab to oxaliplatin-based chemotherapy in first-line treatment of patients with advanced colorectal cancer. While the study showed that Cetuximab increased the response rate measured at 12 weeks after therapy, there was no evidence of benefit in terms of progression-free survival or overall survival in KRAS wild-type patients or even in patients selected by additional mutational analysis of their tumours (Maughan et al., Lancet Oncol 2011;377:2103-2114). Therefore additional biomarkers are required to accurately select patients and differentiate responsive from non-responsive subjects.
Given the intention in the provision of therapy or use of a therapeutic agent to improve clinical outcome through a beneficial response, it would be advantageous to provide a diagnostic test that identifies responsive subjects to the therapy/therapeutic agent being considered. This is particularly advantageous to identify those subjects who are unlikely to benefit and are therefore unnecessarily exposed to toxic side effects of therapeutic agents. Further, it is advantageous to minimise the likelihood of unnecessary expense being incurred through treatment of subjects with therapy/therapeutic agents that are unlikely to provide benefit, and/or minimise the delay in a subject being treated with an alternative drug(s) which might prove more beneficial.